The invention concerns the equipment for mixing and/or plasticizing by mechanical work and the equipment for extrusion of different materials, such as rubber. This invention concerns, in particular, a coating process ensuring protection against wear by contact friction between at least one rotating piece and one stationary piece. It also concerns an extruder screw provided with this coating on at least some of its raised parts.
The threads of the mixing and extrusion screws are, in particular, concerns of the invention, as are the raised parts of the extrusion heads in proximity to the ejection openings of the plasticized material, said parts having the function of ejecting said material.
The area of application of the invention is not solely reserved for rubber, but can be easily extended to plastics and elastomeric compositions or to any material capable of being mixed and extruded after having undergone more or less extensive mechanical work.
In the field of preparation of rubber mixtures, extruders consisting of a metal mixing roll are used, in which at least one metal screw formed by a core surrounded by one or more threads of appropriate shape is rotated.
Thread is understood to mean any raised part on the core, intended mix/shear the base material(s) in order to be able to extrude the mixture obtained through a die of given profile.
The material to be plasticized is introduced in the mixing roll of said machine and undergoes mechanical deformation work (mainly shearing) by the rotation of each internal screw; the dimensions of each screw and of each thread depend on the work necessary for plasticization; the greater that work, the longer the screw in relation to the diameter of its core. Under the action of rotation of the screw, the material tends to present considerable mechanical resistance, which entails more or less substantial bending strains of the core, said core then being able to come in contact with the inner wall of the roll. This contact results in wear of the crown parts of the screw threads, that is, the parts of the threads situated closest to and opposite the inner wall of the mixing roll. The same phenomenon occurs on the raised parts of the extrusion heads intended to push the plasticized material out of the extruder.
This problem of wear of the screw threads is well known and, in order to increase the lifetime of moving pieces that might rub against stationary parts, while maintaining a low cost, the use of extruder screws has been proposed, in which the crown parts of the threads are provided with a metal alloy-base coating, the wear characteristic of which is improved over that of the base material constituting the thread. Alloys called Stellite 6 or Stellite 12 are employed, for example, the hardness and wear characteristics of which make it possible to improve the lifetime of extruder screws appreciably.
A known process of coating consists of making an alloy deposition by welding; a finishing operation is necessary and consists of grinding of the thread surfaces in order to obtain the surface state and dimensions desired. It is to be noted that, taking into account the elevated hardness of the coating, that machining necessitates specific tools and, consequently, the grinding operation is thereby rendered more difficult.
Experience has shown that the deposition of a material of maximum hardness is preferable from the standpoint of wear; however, coatings of elevated hardness, which are very effective on wear, present the disadvantage of being fragile when subjected to mechanical shear stresses like those encountered, for example, in extruders. A so-called scaling phenomenon occurs, consisting of the fact that the surface of the coating of elevated hardness cracks in a plane roughly parallel to the surface on which the stress acts. A scaling is progressively formed in the coating, followed by the tearing away of fragments from said coating; fragment is understood to mean pieces of material of substantially greater size than the size of the particles usually formed by regular wear. Following that weakening of the deposit of elevated hardness, fragments torn away from the coating might be found in the material mixed, which results in the presence of relatively large-sized foreign bodies that can affect the performance of the aforesaid extruded materials on future use. One consequence of such scaling phenomenon is that it is necessary, in order to avoid the presence of deposition fragments in the extruded material, to provide a system of monitoring of the surface state of the threads; said monitoring operation entails, of course, extra costs. Furthermore, the coated pieces need to be renovated rather frequently; restoration or renovation of the deposit is an operation which is certainly practical, but remains, nevertheless, difficult to apply with coating materials of elevated hardness.